The present invention relates to fluid coupling devices, and more particularly, to such devices which are capable of changing between the engaged and disengaged conditions, in response to variations in a predetermined temperature condition.
A fluid coupling device of the type to which the present invention relates typically includes an input coupling member and an output coupling member. The output coupling member cooperates with a cover assembly to define a fluid chamber, and a valve plate separates the chamber into a reservoir chamber and an operating chamber. The input coupling member is rotatably disposed in the operating chamber, and cooperates with the output coupling member to define a viscous shear space, such that torque may be transmitted from the input member to the output member by means of a viscous shear fluid.
The valve plate includes a valving arrangement operable in response to variations in temperature to permit fluid to flow from the reservoir, through the valve plate inlet port into the operating chamber. Typically, such fluid couplings include a discharge port defined by the valve plate and disposed near the outer periphery of the operating chamber, with some form of pumping element, such that a small quantity of fluid is continually pumped from the operating chamber back to the reservoir chamber during normal operation.
Among the problems associated with devices of the type described is the necessity to dissipate heat which is generated as the speed of the input coupling member increases, and the rate of shearing of the viscous fluid increases. In conventional fluid coupling devices, there has been no protection provided to prevent damage to the fluid and the coupling device as the input speed and the fluid temperature increase.
With the advent of smaller automobile engines, operating at relatively higher speeds and temperatures, it becomes increasingly common for the temperature of the fluid in the coupling device to exceed a predetermined maximum temperature. When this occurs, the silicon fluid typically used in such coupling devices undergoes a process in which the fluid "gelatinates". In this process, the fluid first "droops", i.e., there is a physical breakdown of the polymer chains such that the fluid viscosity decreases and the torque transmitting capability of the device decreases substantially. Then, with continued excess fluid temperature, the fluid again begins to cross-link, but does so excessively, and eventually "gels" or becomes almost solid. When the fluid reaches this condition, the coupling device operates as if it were solid, with no slip speed in the engaged mode, and no capability of operating in a disengaged mode.